A prior art circuit for supplying a load with bi-directional current includes a series half-bridge converter comprising a pair of series-connected switches which are alternately switched on to achieve bi-directional current flow through the load.
In order to improve the power factor of the load, the prior art power supply circuit incorporates a boost converter which receives rectified, or d.c., voltage from a full-wave rectifier, which, in turn, is supplied with a.c. voltage and current. The boost converter generates a voltage boosted above the input d.c. voltage on a capacitor of the boost converter ("the boost capacitor"), which supplies the d.c. bus voltage for powering the mentioned series half-bridge converter. The prior art boost converter includes a dedicated switch ("the boost switch") which repetitively connects an inductor of the boost converter ("the boost inductor") to ground and thereby causes current flow in the such inductor, and hence energy storage in the inductor. The energy stored in the boost inductor is then directed to the boost capacitor, to maintain a desired bus voltage on such capacitor.
In the operation of the prior art boost converter, the energy stored in the boost inductor is completely discharged into the boost capacitor prior to the boost switch again connecting the boost inductor to ground. Operation of the boost converter as described, i.e. with complete energy discharge of the boost inductor, is known as operation in the discontinuous mode of energy storage.
The described prior art power supply circuit has been found to achieve a typical power factor of about 0.98; it has also been found to result in a total harmonic distortion of the a.c. input current supplied to the full-wave rectifier of less than about 13%, which distortion arises from the power supply circuit drawing an a.c. current that departs from a perfect sinusoidal waveform.
One drawback of the described prior art circuit is that its overall gain typically has a wide variation, especially when powering such loads as a fluorescent lamp whose loading varies considerably in normal operation. This can cause a large ripple in the voltage applied to the load, with the ratio of actual voltage to r.m.s. voltage exceeding 1.7. It would be desirable if such ratio, known as the crest factor, could be maintained below about 1.7. This would reduce fatigue on a fluorescent lamp load, for instance, that would otherwise shorten lamp life.